3 files changed, 54 insertions, 7 deletions

diff --git a/video/gtk4/src/sink/frame.rs b/video/gtk4/src/sink/frame.rs
index 4a53e1a07..d5223a613 100644
--- a/video/gtk4/src/sink/frame.rs
+++ b/video/gtk4/src/sink/frame.rs
@@ -42,6 +42,7 @@ pub(crate) struct Texture {
     pub width: f32,
     pub height: f32,
     pub global_alpha: f32,
+    pub has_alpha: bool,
 }
 
 struct FrameWrapper(gst_video::VideoFrame<gst_video::video_frame::Readable>);
@@ -149,6 +150,7 @@ impl Frame {
 
         let width = self.frame.width();
         let height = self.frame.height();
+        let has_alpha = self.frame.format_info().has_alpha();
         let (texture, pixel_aspect_ratio) = {
             #[cfg(not(any(target_os = "macos", target_os = "windows", feature = "gst_gl")))]
             {
@@ -183,9 +185,11 @@ impl Frame {
             width: width as f32 * pixel_aspect_ratio as f32,
             height: height as f32,
             global_alpha: 1.0,
+            has_alpha,
         });
 
         for overlay in self.overlays {
+            let has_alpha = overlay.frame.format_info().has_alpha();
             let (texture, _pixel_aspect_ratio) =
                 video_frame_to_memory_texture(overlay.frame, cached_textures, &mut used_textures);
 
@@ -196,6 +200,7 @@ impl Frame {
                 width: overlay.width as f32,
                 height: overlay.height as f32,
                 global_alpha: overlay.global_alpha,
+                has_alpha,
             });
         }
 
diff --git a/video/gtk4/src/sink/paintable/imp.rs b/video/gtk4/src/sink/paintable/imp.rs
index 6763ce5a4..dedfcdf18 100644
--- a/video/gtk4/src/sink/paintable/imp.rs
+++ b/video/gtk4/src/sink/paintable/imp.rs
@@ -11,7 +11,7 @@
 
 use gtk::prelude::*;
 use gtk::subclass::prelude::*;
-use gtk::{gdk, glib, graphene};
+use gtk::{gdk, glib, graphene, gsk};
 
 use crate::sink::frame::{Frame, Texture};
 
@@ -28,11 +28,25 @@ static CAT: Lazy<gst::DebugCategory> = Lazy::new(|| {
     )
 });
 
-#[derive(Default, Debug)]
+#[derive(Debug)]
 pub struct Paintable {
     paintables: RefCell<Vec<Texture>>,
     cached_textures: RefCell<HashMap<usize, gdk::Texture>>,
     gl_context: RefCell<Option<gdk::GLContext>>,
+    premult_shader: gsk::GLShader,
+}
+
+impl Default for Paintable {
+    fn default() -> Self {
+        Self {
+            paintables: Default::default(),
+            cached_textures: Default::default(),
+            gl_context: Default::default(),
+            premult_shader: gsk::GLShader::from_bytes(&glib::Bytes::from_static(include_bytes!(
+                "premult.glsl"
+            ))),
+        }
+    }
 }
 
 #[glib::object_subclass]
@@ -145,14 +159,31 @@ impl PaintableImpl for Paintable {
                 width: paintable_width,
                 height: paintable_height,
                 global_alpha,
+                has_alpha,
             } in &*paintables
             {
                 snapshot.push_opacity(*global_alpha as f64);
-                snapshot.append_texture(
-                    texture,
-                    &graphene::Rect::new(*x, *y, *paintable_width, *paintable_height),
-                );
-                snapshot.pop();
+
+                let bounds = graphene::Rect::new(*x, *y, *paintable_width, *paintable_height);
+
+                // Only premultiply GL textures that expect to be in premultiplied RGBA format.
+                let do_premult = texture.is::<gdk::GLTexture>() && *has_alpha;
+                if do_premult {
+                    snapshot.push_gl_shader(
+                        &self.premult_shader,
+                        &bounds,
+                        gsk::ShaderArgsBuilder::new(&self.premult_shader, None).to_args(),
+                    );
+                }
+
+                snapshot.append_texture(texture, &bounds);
+
+                if do_premult {
+                    snapshot.gl_shader_pop_texture(); // pop texture appended above from the shader
+                    snapshot.pop(); // pop shader
+                }
+
+                snapshot.pop(); // pop opacity
             }
         } else {
             gst::trace!(CAT, imp: self, "Snapshotting black frame");
diff --git a/video/gtk4/src/sink/paintable/premult.glsl b/video/gtk4/src/sink/paintable/premult.glsl
new file mode 100644
index 000000000..f2ab7ec55
--- /dev/null
+++ b/video/gtk4/src/sink/paintable/premult.glsl
@@ -0,0 +1,11 @@
+uniform sampler2D u_texture1;
+
+void mainImage(
+    out vec4 fragColor,
+    in vec2 fragCoord,
+    in vec2 resolution,
+    in vec2 uv
+) {
+  fragColor = GskTexture(u_texture1, uv);
+  fragColor.rgb = fragColor.rgb * fragColor.a;
+}